At 11:38 AM on January 28, 1986, the space shuttle Challenger lifted off from its launch pad at Cape Canaveral. Seventy-four seconds later, it was ten miles high and rising. Then it blew up. The launch was televised, so news of the accident spread quickly. Eight minutes after the explosion, the first story hit the Dow Jones News Wire.

The stock market did not pause to mourn. Within minutes, investors started dumping the stocks of the four major contractors who had participated in the Challenger launch; Rockwell International, which built the shuttle and its main engines; Lockheed, which managed ground support; Martin Marietta, which manufactured the ship’s external fuel tank and Morton Thiokol, which built the solid-fuel booster rocket. Twenty-one minutes after the explosion, Lockheed’s stock was down 5 percent, Martin Marietta’s was down 3 percent, and Rockwell was down 6 percent.

Morton Thiokol’s stock was hit hardest of all. As the finance professors Michael T. Maloney and J. Harold Mulherin report in their fascinating study of the market’s reaction to the Challenger disaster so many investors were trying to sell Thiokol stock and so few people were interested in buying it that a trading halt was called almost immediately. When the stock started trading again, almost an hour after the explosion, it was down 6 percent. By the end of the day, its decline had almost doubled, so that at market close, Thiokol’s stock was down nearly 12 percent. By contrast, the stocks of the three other firms started to creep back up, and by theend of the day their value had fallen only around 3 percent.

What this means is that the stock market had, almost immediately labeled Morton Thiokol as the company that was responsible for the Challenger disaster. The stock market is, at least in theory a machine for calculating the present value of all the “free cash flow” a company will earn in the future, (Free cash flow is the money that’s left over after a company has paid all its bills and its taxes, has accounted for depreciation, and has invested in the business. It’s the money you’d get to take home and put in the bank if you were the sole owner of the company) The steep decline in Thiokol’s stock price—especially compared with the slight declines in the stock prices of its competitors—was an unmistakable sign that investors believed that Thiokol was responsible, and that the consequences for its bottom line would he severe.

As Maloney and Mulherin point out, though, on the day of the disaster there were no public comments singling out Thiokol as the guilty party. While the New York Times article on the disaster that appeared the next morning did mention two rumors that had been making the rounds, neither of the rumors implicated Thiokol, and the Times declared, "There are no clues to the cause of the accident.”

Regardless, the market was right. Six months after the explosion, the Presidential Commission on the Challenger revealed that the 0-ring seals on the booster rockets made by Thiokol—seals that were supposed to prevent hot exhaust gases from escaping—became less resilient in cold weather, creating gaps that allowed the gases to leak out. (The physicist Richard Feynman famously demonstrated this at a congressional hearing by dropping an 0-ring in a glass of ice water. When he pulled it out, the drop in temperature had made it brittle.) In the case of the Challenger, the hot gases had escaped and burned into the main fuel tank, causing the cataclysmic explosion. Thiokol was held liable for the accident. The other companies were exonerated.

In other words, within a half hour of the shuttle blowing up, the stock market knew what company was responsible. To be sure, this was a single event, and it’s possible that the market's singling out of Thiokol was just luck. Or perhaps the company’s business seemed especially susceptible to a downturn in the space program. Possibly the trading halt had sent a signal to investors to be wary These all are important cautions, but there is still something eerie about what the market did. That's especially true because in this case the stock market was working as a pure weighing machine, undistorted by the factors—media speculation, momentum trading, and Wall Street hype—that make it a peculiarly erratic mechanism for aggregating the collective wisdom of investors. That day, it was just buyers and sellers trying to figure out what happened and getting it right.

How did they get it right? That’s the question that Maloney and Mulherin found so vexing. First, they looked at the records of insider trades to see if Thiokol executives, who might have known that their company was responsible, had dumped stock on January 28. They hadn’t. Nor had executives at Thiokol’s competitors, who might have heard about the 0-rings and sold Thiokol’s stock short. There was no evidence that anyone had dumped Thiokol stock while buying the stocks of the other three contractors (which would have been the logical trade for someone with inside information). Savvy insiders alone did not cause that first-day drop in Thiokol’s price. It was all those investors—most of them relatively uninformed—who simply refused to buy the stock.

But why did they not want Thiokol’s stock? Maloney and Mulherin were finally unable to come up with a convincing answer to that question. In the end, they assumed that insider information was responsible for the fail in Thiokol’s price, but they could not explain how. Tellingly, they quoted the Cornell economist Maureen O’Hara, who has said, “While markets appear to work in practice, we are not sure how they work in theory”

Maybe. But it depends on what you mean by “theory”. If you strip the story down to its basics, after all, what happened that January day was this: a large group of individuals (the actual and potential shareholders of Thiokol’s stock, and the stocks of its competitors) was asked a question—”how much less are these four companies worth now that the Challenger has exploded?”—that had an objectively correct answer. Those are conditions under which a crowds average estimate—which is, dollar weighted, what a stock price is—h likely to be accurate. Perhaps someone did, in fact, have inside knowledge of what had happened to the 0-rings. But even if no one did, it's plausible that once you aggregated all the bits of information about the explosion that all the traders in the market had in their heads that day, it added up to something close to the truth. As was true of those who helped John Craven find the Scorpion, even if none of the traders was sure that Thiokol was responsible, collectively they were certain it was.

The market was smart that day because it satisfied the four conditions that characterize wise crowds: diversity of opinion (each person should have some private information, even if it’s just an eccentric interpretation of the known facts), independence (people’s opinions are not determined by the opinions of those around them), decentralization (people are able to specialize and draw on local knowledge), and aggregation (some mechanism exists for turning private judgments into a collective decision). If a group satisfies those conditions, its judgment is likely to be accurate. Why? At heart, the answer rests on a mathematical truism. If you ask a large enough group of diverse, independent people to make a prediction or estimate a probability and then average those estimates, the errors each of them makes in coming up with an answer will cancel themselves out. Each person’s guess, you might say, has two components: information and error. Subtract the error, and you’re left with the information.

Now, even with the errors canceled out, it’s possible that a group’s judgment will be bad. For the group to be smart, there has to be at least some information in the “information” part of the “information minus error” equation. (If you’d asked a large group of children to buy and sell stocks in the wake of the Challenger disaster it’s unlikely they would have picked out Thiokol as the culprit.) What is striking, though—and what makes a phrase like “the wisdom of crowds” meaningful—is just how much information a group’s collective verdict so often contains. In cases like Francis Galton’s experiment or the Challenger explosion, the crowd is holding a nearly complete picture of the world in its collective brain.

Perhaps this isn’t surprising. After all, we are the products of evolution, and presumably we have been equipped to make sense of the world around us. But who knew that, given the chance, we can collectively make so much sense of the world. After all, think about what happens if you ask a hundred people to run a 100-meter race, and then average their times. The average time will not be better than the time of the fastest runners. It will be worse. It will be a mediocre time. But ask a hundred people to answer a question or solve a problem, and the average answer will often be at least as good as the answer of the smartest member. With most things, the average is mediocrity. With decision making, it’s often excellence. You could say it’s as if we've been programmed to be collectively smart.